1. Field of the Invention
The present invention relates to a punch for a punching press to perform a nibbling operation on a sheet material, a nibbler assembly and a method of forming an elongated hole in the sheet material.
2. Description of Related Art
To nibble a sheet material with the use of a punching press, a method of continuously processing a number of punched holes has been well known, in which the die is a punching die of a type having a rectangular transverse sectional shape. According to this known method, a problem arises such that ends of overlap areas of holes which are continuously punched are such that, as shown in FIGS. 20A and 20B, an upper portion of a punched surface 100 of a sheet material W tends to have a projection-shaped seam 102 with no roll-over 101 formed. It is to be noted that, as shown in FIG. 20C, the punched surface 100 formed by a punching process represents a roll-over 101, a sheared surface 103 and a fractured surface 104 in this order with respect to the side in which a punch is advanced.
As a punch processing method effective to eliminate the above described seam 102, a method has been suggested (in, for example, Japanese Patent No. 3960066) in which with the use of a nibbler assembly having a cutting blade section that includes a bottom surface inclined relative to the level surface, the sheet material is not punched out but is cut at the cutting blade section and the subsequent cutting is continued from the previous cutting. According to such processing method in which the cuttings are successively applied, punched surfaces can be continuously finished beautifully. The nibbler assembly employed in the practice of the above described suggested method has a punching blade section separate from the cutting blade section and, by means of this punching blade section, an inner side portion of the cutting formed by the cutting blade section during the previous punching process is punched out.
To use the above described conventional nibbler assembly, an access opening needs to be formed in the sheet material beforehand, which access opening is necessitated to enable a blade portion of the punch to be inserted therethrough when a cutting is worked on the sheet material at the initial process of nibbling. For this reason, at the end of the overlap area between the access opening and the initial cutting, a projection-shaped seam 102 (best shown in FIGS. 20A and 20B) is inevitably left therein.
Also, the above described conventional nibbler assembly is, in the final step of nibbling, incapable of punching off an inner side portion of the cutting, formed during the previous punching process, with a portion left uncut. That is because the inner side portion of the cutting is downwardly bent and, therefore, such a bent inner side portion interferes with a die. For example, in an inner periphery processing such as a processing of a corner portion of, for example, a window hole, when the corner portion is to be punched out with the use of the standard punch, unless a portion of the cutting is left in a final portion of an elongated hole formed by nibbling, a projection-shaped seam 102 (best shown in FIGS. 20A and 20B) tends to be left in one end of an overlap area between the elongated hole, formed by nibbling, and a punched hole formed by the standard punch in a manner similar to that described hereinbefore.